The measurement of proximal flow in the breathing circuit of a ventilating system in the critical care environment can often be challenging, particularly given the high humidity and secretions present and the length of mechanical ventilation that can last from days to weeks. Timing the start and end of the inspiratory and expiratory phases, for example, depends upon the observed flow speed in the breathing circuit between the patient's lungs and the ventilator gas flow generator.
Due to their robustness, differential pressure based flow sensors are often used in clinical environments. Differential pressure flow sensors incorporate some type of restriction (point orifice, variable flap, vena constriction, annular obstruction, target or linear flow restrictor) that generates a pressure difference across the sensor. Flexible tubing, attached to either side of the flow obstruction, transmits the differential pressure signal from the restriction located on the breathing circuit to a sensitive pressure sensor located inside a monitor, or monitoring module, at the bedside.
To maintain performance and function in this environment over time, differential pressure based respiratory measurement systems, implementable for example in the monitoring module, generally include zeroing and purging functions. Periodic zeroing, performed by exposing the two sides of the differential pressure sensors to the same pressure for a short period of time, is required. This is because pressure sensors drift due to both intrinsic and extrinsic factors including changes in temperature. High humidity in the breathing circuit often leads to condensation of moisture in the pressure transmission tubing eventually resulting in a damped and distorted pressure signal (e.g., reduced accuracy) if not cleared. Therefore, pressure transmission tubes are periodically purged with a source of gas (either air or inspiratory gas) to reduce the adverse effects of condensate on pressure and flow measurements.
The complexity associated with the valves and interconnections required for zeroing and purging functions has resulted in conventional respiratory measurement systems with “bulky”, multi-piece designs, which are difficult to assemble due to many individual pneumatic connections.
Additionally, the number of connections between different components results in a greater potential for leaks at these interfaces.